1. Field of the Invention
The present invention relates to a SiC sintered body.
2. Discussion of the Background
Pressureless sintering is a preferred method for manufacturing a SiC sintered body having a complicated shape, not only because this method is simple and inexpensive, but because a sintered body having a shape closer to that desired can be obtained, requiring less processing to finish the product as compared to the hot-press sintering method. In pressureless sintering, various types of sintering aids are used to enhance the atomic diffusion of elements.
An example of such a sintering aid is carbon. When carbon is used any oxide film (SiO.sub.2) formed is reduced and eliminated as formulated below: EQU SiO.sub.2 +3C.fwdarw.SiC+2CO.uparw.
At the same time, the carbon aid also makes the SiC sintered body dense. This is because Si vacancies are formed in the SiC fine crystals when carbon is added, and these vacancies serve to promote Si atom diffusion, which is usually relatively low, up to the level of that of carbon.
However, the lattice diffusion promoting effect which can be obtained by adding the carbon is not enough to achieve the desired densification of a SiC sintered body. In order to further enhance lattice diffusion, boron (B), aluminum (Al), or a rare earth element can be used in addition to carbon. For example, use of B-C family aids is set forth in Published Unexamined Japanese Patent Application (PUJPA) No. 51-148712 and PUJPA No. 52-6716. Further, PUJPA Nos. 54-118411 and 57-160970 disclose use of Al-C and rare earth-C family auxiliaries, respectively. These auxiliaries help to prepare a dense sintered body using the pressureless sintering method, and from this dense sintered body a complicated-shaped SiC sintered body can be easily obtained. In addition, the sintering process in the above-mentioned case is a solid phase sintering, which does not produce a liquid phase; therefore the fast fracture strength of the sintered body does not deteriorate until the temperature reaches 1500.degree. C. where, for example, an Al-C family aid is used. Consequently, a SiC sintered body produced by this sintering method can be used as a high-temperature-structural part in a gas turbine.
Such a gas turbine, however, is required to be driven at a temperature higher than 1500.degree. C. in order to improve its energy efficiency. The properties of the above-mentioned conventional SiC sintered body is not yet good enough to meet such requirements.
More specifically, if a SiC sintered body prepared through the use of a B-C, Al-C, or rare earth-C family auxiliary is employed as a high temperature structural material, e.g. in a gas turbine, under high temperature conditions over 1500.degree. C., the life of the sintered body is shortened due to deterioration of the mechanical properties thereof, caused by high temperature corrosive oxidation.
In consideration of the above problem, a SiC sintered body which is mechanically strong at temperatures over 1500.degree. C. and has excellent high temperature corrosive oxidation resistance is needed.